It is not a rocket science, if you really think about it. First, capitalism is not a reified entity, but a system of relations, so it does not need to be torn down like an old building and replaced with a brand new structure. Changing capitalism is identifying those relationships that have negative byproducts (the negative critique), and replecing them with new relationships free of those byproduct - the latter being more like sex than economics, a trial and error process rather than a blueprint a priori conceived by geeks with no life.
So one of the problems of capitalism is private ownership of the means of production, which promotes the "keep up with the Joneses" dynamics - that is, production level expands to fill the available prodctive capacity (on the fear that competitors would do that before us), rather than human needs. Hence relentless production of goods for theoir exchange value, while genuine human needs (such as hunger) are still unmet.
A related problem is free riding that private ownership of the means of production facilitates - that is, dumping some of the costs on someone else while keeping profits for oneself. Hence wasteful, polluting technologies, such as auto-based transportation of suburban housing that are profitable to a few, but a burden to those excluded from "the loop."
Negative critique identifies those problems and their source - private ownership of the means of production. Positive critique may want to replace that particularly problematic relationship with a different one that would eliminate the above identified adverse effects. For example, public ownership of the means of production. It will take away the incentive from "keeping up with Joneses" expansion of production - which in turn will reduce the working time, thus allowing the producers to "fish in the morning and write peopetry in the evening." The level of production inthat scenario will thus be determined by how much fishing and poetry writing the workers are willing to scarifice for a new SUV, rather than factopry owners' drive to outcompete other factory owners and in so doing, employing the army of brainwashers to push their products as a "life necessity."
Moreover, poblic ownership of the means of production would take away the incentive form free riding by externalizing some of the costs. Again the level of production and pollution woul dbe determined by the workers willingness to sacrfice fishing in a clean water, and breathe unpoluted air while writing their poetry - rather than factory owner drive to outcompete other factory owners.
So as you can see, it is not working for a socially owned unit of production instead of the capitalist per se - but the absence of some very real side effects trhat capitalsi property ownership produces - lack of control of worker's time, ineffcient technologies, polluted environment, etc. that matters.
wojtek